Wood Handbook, Wood as an Engineering Material

for wood or steel side members, which is taken into account
by thickness and dowel bearing strength parameters. The
equations are also valid for various load-to-grain directions,
which are taken into account by the Kq and Fe parameter.

The dowel bearing strength is a material property not gen-
erally familiar to structural designers. The dowel bearing
strength of the wood members is determined from tests that
relate species specific gravity and dowel diameter to bearing
strength. Empirical equations for these relationships are as
follows:

Parallel to grain

(metric) (8–17a)

(inch–pound) (8–17b)

Perpendicular to grain

(metric) (8–18a)

(inch–pound) (8–18b)

where Fe is dowel bearing strength (MPa, lb in–2), G specific
gravity based on ovendry weight and volume, and D bolt
diameter (mm, in.).

For other angles of loading, the dowel bearing strength

values for use in the yield model are determined by the

Hankinson equation, where P and Q are the values of dowel

bearing parallel and perpendicular to grain, respectively.

Connector Joints

Several types of connectors have been devised that increase

joint bearing and shear areas by utilizing rings or plates

around bolts holding joint members together. The primary

load-carrying portions of these joints are the connectors; the

bolts usually serve to prevent transverse separation of the

members but do contribute some load-carrying capacity.

The strength of the connector joint depends on the type and

size of the connector, the species of wood, the thickness and

width of the member, the distance of the connector from the

end of the member, the spacing of the connectors, the direc-

tion of application of the load with respect to the direction

of the grain of the wood, and other factors. Loads for wood

joints with steel connectors—split ring (Fig. 8–16) and shear

plate (Fig. 8–17)—are discussed in this section. These con-

nectors require closely fitting machined grooves in the wood

members.

Parallel-to-Grain Loading

Tests have demonstrated that the density of the wood is a

controlling factor in the strength of connector joints. For

split-ring connectors, both maximum load and proportional

limit load parallel to grain vary linearly with specific grav-

ity (Figs. 8–18 and 8–19). For shear plates, the maximum

load and proportional limit load vary linearly with specific

gravity for the less dense species (Figs. 8–20 and 8–21). In

the higher density species, the shear strength of the bolts

becomes the controlling factor. These relations were

Chapter 8 Fastenings

Table 8–16. The 5% offset yield lateral strength(Z)

for three-member bolted joints

Mode

Z value for three-

member bolted joint

Mode Im K

DtmFem

Mode Is K

2 DtsFes

Mode IIIs R)K

kDtF

e

4 s em

( 2

2



Mode IV 3 ( 1 )

2 2

e

(^2) em yb
R
F F
K
D
θ 
Definitions
D nominal bolt diameter, mm (in.)
Fem dowel bearing strength of main (center)
member, MPa (lb in–2)
Fes dowel bearing strength of side members,
MPa (lb in–2)
Fyb bending yield strength of bolt, MPa (lb in–2)
K 1 + /360
tm thickness of main (center) member, mm (in.)
ts thickness of side member, mm (in.)
Z nominal single bolt design value
 angle of load to grain (degrees)
Re = Fem/Fes
2
ems
yb e^2
e
4 e
3
2 ( 1 )^2 (^2 )
1
F t
F R D
R
k   R  
Figure 8–16. Joint with split-ring connector showing
connector, precut groove, bolt, washer, and nut.